This invention relates to a caustic solution for etching aluminum and more particularly, it relates to a method of controlling the dissolved aluminum content of the solution.
Caustic etching is employed in the aluminum finishing industry for both functional and decorative purposes. For example, all aluminum used for anodizing purposes is etched initially in order to provide a uniform surface appearance. During the etching process, aluminum dissolves and combines with the sodium hydroxide to form sodium aluminate according to the reaction EQU 2Al + 2NaOH + 2H.sub.2 O .fwdarw. 2NaAlO.sub.2 + 3H.sub.2 .uparw.
through use, the effective NaOH content of the solution decreases and the concentration of NaAlO.sub.2 increases. Eventually, the solution becomes saturated with NaAlO.sub.2, at which point aluminum hydroxide will precipitate in accordance with the reaction EQU 2NaAlO.sub.2 + 4H.sub.2 O .fwdarw. 2Al(OH).sub.3 .dwnarw. + 2NaOH
it can be seen from both chemical equations that the overall reaction is between aluminum and water yielding hydrogen and aluminum hydroxide. Thus, the caustic solution should only consume NaOH in an amount equivalent to that which is dragged out by the work loads since precipitation of Al(OH).sub.3 liberates or frees sodium hydroxide again. However, there are problems which arise when an etch solution is used in this way. For example, when Al(OH).sub.3 precipitates, it forms a hard rock-like scale on the tank and on heat exchanger walls which scale is very difficult to remove. Also, for purposes of keeping the etch rate constant, fresh NaOH has to be added to make up for that consumed as NaAlO.sub.2. To avoid such problems in conventional practice, the etch solution must be discarded before precipitation occurs. Thus, the etch tank must be drained and cleaned frequently, causing costly production downtime as well as using NaOH very inefficiently.
To decrease the number of times the etch solution must be drained, a chelating agent such as sodium gluconate may be added to retard spontaneous precipitation of Al(OH).sub.3. However, as noted above, to maintain consistent operation, NaOH must be added periodically. Thus, the etch continually increases in total NaOH and dissolved aluminum. Eventually, the etch solution becomes very concentrated, resulting in a viscous solution which does not etch uniformly. At this point, the solution normally has to be discarded, again resulting in wasted NaOH and in production downtime.
In an approach to minimize these problems, often caustic solutions are maintained at an equilibrium composition by continually discharging etch solution and replenishing such with fresh NaOH, water, and other additives. While this method obviates the production downtime problem, it consumes sodium hydroxide in relation to aluminum processed instead of the minimal loss due to removing the work load. All of this inefficiently utilized NaOH plus the aluminum thereby dissolved during etching is ultimately lost to waste treatment and disposal.
The present invention eliminates the problems attendant the operation of a caustic solution for etching aluminum by providing a method which controls the dissolved aluminum content of the caustic solution. The method of the present invention substantially eliminates any need to dump the solution for reason of its becoming inoperative. Also, the method, as well as eliminating downtime for preparing new solutions, can result in as much as a 90% reduction in the consumption of sodium hydroxide, when compared to conventional practices. In addition, in the present invention, NaOH can be liberated or regenerated at a rate substantially equal to its consumption during etching thereby ensuring a relatively constant level of NaOH in the etching solution.